For application to electric and electronic appliances for household use, construction materials and vehicles, heretofore widely used are chromate-treated steel sheets that are fabricated by treating the surfaces of zinc-plated steel sheets or aluminium-plated steel sheets with a surface-treating agent comprising, as the essential ingredient, chromic acid, bichromic acid or its salt for improving the corrosion resistance (white rust resistance, red rust resistance) of the steel sheets. The chromate treatment is a relatively simple and economical method for forming films of good corrosion resistance.
In such chromate treatment, used is hexavalent chromium, a substance of which the use is controlled for antipollution. The hexavalent chromium is post-treated in a closed system, in which it is completely reduced and collected, and is therefore not discharged out into the natural world, and, in addition, the chromium release from chromate films can be almost zero owing to the sealing action of the organic films. Accordingly, the environment and human bodies are not substantially polluted by hexavalent chromium. However, from the recent global environmental problems, a tendency toward self-imposed restraints on the use of heavy metals including hexavalent chromium is increasing. In addition, for protecting the environment from pollution by dumped shredder dust of wastes, a movement of not using as much as possible heavy metals or reducing them in products has begun.
Given that situation, many proposals of chromium-free technology with no chromate treatment have been made for preventing white rust formation on zinc-plated steel sheets. For example, steel sheets are dipped in, or coated with or electrolyzed in a solution of an inorganic compound, an organic compound, an organic polymer material or a combination of any of them to thereby form a thin film thereon.
Concretely, the following methods may be employed.    (1) A method of dipping steel sheets in a processing solution that comprises a polyphenol-carboxylic acid such as tannic acid and a silane coupling agent, or applying the processing solution to them to thereby form a film on them (e.g., JP-A 7-216268, Japanese Patent 2,968,959).    (2). A method of using a processing solution that is prepared by adding a polyphenol-carboxylic acid such as tannic acid or a phosphoric acid compound to an organic resin, for forming a film (e.g., JP-A 8-325760, 2000-34578, 2000-199076, 2000-248380).    (3) A method of applying a processing solution that is prepared by mixing an organic resin and a silane coupling agent, onto steel sheets (e.g., JP-A 10-80664).
According to the above-mentioned method (1), steel sheets are treated with an aqueous solution that comprises a polyphenol-carboxylic acid, a silane coupling agent and optionally a metal ion. One example of the method is described in JP-A 7-216268. The method ensures good adhesiveness of the film to steel sheets, but is defective in that the corrosion resistance of the film is not enough.
For the method (2), for example, JP-A 8-325760 discloses a method of processing steel sheets with a processing solution that comprises a polyphenol-carboxylic acid, an organic resin and a metal ion. JP-A 2000-34578 discloses a method of dipping steel sheets in a processing solution that comprises an organic resin and a phosphoric acid compound, or applying the processing solution to steel sheets, followed by drying the thus-processed steel sheets. The protective films formed from these processing solutions could be effective in some degree for improving the corrosion resistance of steel, but they could not attain high-level corrosion resistance like chromate treatment.
For the method (3), JP-A 10-80664 discloses a method of applying a processing solution that is prepared by mixing an organic resin and a silane coupling agent, to steel to thereby form a film thereon. In the processing solution, however, the silane coupling agent is not satisfactorily active on an inert metal surface and therefore the adhesiveness and the corrosion resistance of the film formed are unsatisfactory.